Semiconductor structure and method of forming the same

ABSTRACT

A semiconductor structure includes a first III-V compound layer. A second III-V compound layer is disposed on the first III-V compound layer and different from the first III-V compound layer in composition. A carrier channel is located between the first III-V compound layer and the second III-V compound layer. A source feature and a drain feature are disposed on the second III-V compound layer. A gate electrode is disposed over the second III-V compound layer between the source feature and the drain feature. A carrier channel depleting layer is disposed on the second III-V compound layer. The carrier channel depleting layer is deposited using plasma and a portion of the carrier channel depleting layer is under at least a portion of the gate electrode.

TECHNICAL FIELD

The disclosure relates generally to a semiconductor structure and, more particularly, to a high electron mobility transistor (HEMT) and method for forming a high electron mobility transistor.

BACKGROUND

In semiconductor technology, due to their characteristics, Group III-Group V (or III-V) semiconductor compounds are used to form various integrated circuit devices, such as high power field-effect transistors, high frequency transistors, or high electron mobility transistors (HEMTs). A HEMT is a field effect transistor incorporating a junction between two materials with different band gaps (i.e., a heterojunction) as the channel instead of a doped region, as is generally the case for metal oxide semiconductor field effect transistors (MOSFETs). In contrast with MOSFETs, HEMTs have a number of attractive properties including high electron mobility and the ability to transmit signals at high frequencies, etc.

From an application point of view, enhancement-mode (E-mode) HEMTs have many advantages. E-mode HEMTs allow elimination of negative-polarity voltage supply, and, therefore, reduction of the circuit complexity and cost. Despite the attractive properties noted above, a number of challenges exist in connection with developing III-V semiconductor compound-based devices. Various techniques directed at configurations and materials of these III-V semiconductor compounds have been implemented to try and further improve transistor device performance.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the following detailed description and the accompanying figures. It is emphasized that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.

FIG. 1A is a cross-sectional view of a semiconductor structure having a high electron mobility transistor (HEMT) according to one or more embodiments of this disclosure.

FIG. 1B is a cross-sectional view of a semiconductor structure having a HEMT according to another embodiment of this disclosure.

FIG. 2 is a flowchart of a method of forming a semiconductor structure having a HEMT according to one or more embodiments of this disclosure.

FIGS. 3 to 8 are cross-sectional views of a semiconductor structure having a HEMT at various stages of manufacture according to one embodiment of the method of FIG. 2.

DETAILED DESCRIPTION

The making and using of illustrative embodiments are discussed in detail below. It should be appreciated, however, that the disclosure provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative and do not limit the scope of the disclosure.

A plurality of semiconductor chip regions is marked on the substrate by scribe lines between the chip regions. The substrate will go through a variety of cleaning, layering, patterning, etching and doping steps to form integrated circuits. The term “substrate” herein generally refers to the bulk substrate on which various layers and device structures may be formed. In some embodiments, the bulk substrate includes silicon or a compound semiconductor, such as GaAs, InP, Si/Ge, or SiC. Examples of such layers include dielectric layers, doped layers, polysilicon layers or conductive layers. Examples of device structures include transistors, resistors, and/or capacitors, which may be interconnected through an interconnect layer to additional integrated circuits.

FIGS. 1A and 1B are cross-sectional views of semiconductor structures 100 and 101, each semiconductor structure having a high electron mobility transistor (HEMT) according to one or more embodiments of this disclosure.

Referring to FIG. 1A, the semiconductor structure 100 having a HEMT is illustrated. The semiconductor structure 100 includes a substrate 102. In some embodiments, the substrate 102 includes a silicon carbide (SiC) substrate, sapphire substrate or a silicon substrate.

The semiconductor structure 100 also includes a heterojunction formed between two different semiconductor material layers, such as material layers with different band gaps. For example, the semiconductor structure 100 includes a non-doped narrow-band gap channel layer and a wide-band gap n-type donor-supply layer. In at least one embodiment, the semiconductor structure 100 includes a first III-V compound layer (or referred to as a channel layer) 104 formed on the substrate 102 and a second III-V compound layer (or referred to as a donor-supply layer) 106 formed on the channel layer 104. The channel layer 104 and the donor-supply layer 106 are compounds made from the III-V groups in the periodic table of elements. However, the channel layer 104 and the donor-supply layer 106 are different from each other in composition. The channel layer 104 is undoped or unintentionally doped (UID). In the present example of the semiconductor structure 100, the channel layer 104 includes a gallium nitride (GaN) layer (also referred to as the GaN layer 104). The donor-supply layer 106 includes an aluminum gallium nitride (AlGaN) layer (also referred to as AlGaN layer 106). The GaN layer 104 and AlGaN layer 106 directly contact each other. In another example, the channel layer 104 includes a GaAs layer or InP layer. The donor-supply layer 106 includes an AlGaAs layer or an AlInP layer.

The GaN layer 104 is undoped. Alternatively, the GaN layer 104 is unintentionally doped, such as lightly doped with n-type dopants due to a precursor used to form the GaN layer 104. In one example, the GaN layer 104 has a thickness in a range from about 0.5 micron to about 10 microns.

The AlGaN layer 106 is intentionally doped. In one example, the AlGaN layer 106 has a thickness in a range from about 5 nanometers (nm) to about 50 nanometers.

The band gap discontinuity exists between the AlGaN layer 106 and the GaN layer 104. The electrons from a piezoelectric effect in the AlGaN layer 106 drop into the GaN layer 104, creating a very thin layer 108 of highly mobile conducting electrons in the GaN layer 104. This thin layer 108 is referred to as a two-dimensional electron gas (2-DEG), forming a carrier channel (also referred to as the carrier channel 108). The thin layer 108 of 2-DEG is located at an interface of the AlGaN layer 106 and the GaN layer 104. Thus, the carrier channel has high electron mobility because the GaN layer 104 is undoped or unintentionally doped, and the electrons can move freely without collision with the impurities or substantially reduced collisions.

The semiconductor structure 100 also includes a source feature and a drain feature disposed on the AlGaN layer 106 and configured to electrically connect to the carrier channel 108. Each of the source feature and the drain feature comprises a metal feature 112. In one example, the metal feature 112 is free of Au and comprises Al, Ti, or Cu.

The semiconductor structure 100 further includes a dielectric cap layer 110 disposed on a top surface of the AlGaN layer 106 not occupied by the metal features 112. The dielectric cap layer 110 further includes an opening that exposes a portion of the AlGaN layer 106 for the gate electrode formation. The dielectric cap layer 110 protects the underlying AlGaN layer 106 from damage in the following process having plasma.

A protection layer 114 is disposed on top surfaces of the metal features 112 and the dielectric cap layer 110. The protection layer 114 further includes an opening that aligns with the opening in the dielectric cap layer 110. The combined opening of the opening in the protection layer 114 and the opening in the dielectric cap layer 110 exposes the portion of the AlGaN layer 106 for the gate electrode formation.

The semiconductor structure 100 further includes a carrier channel depleting layer 126 deposited using plasma on the protection layer 114, which is disposed along an interior surface of the combined opening and on the exposed portion of the AlGaN layer 106. The carrier channel depleting layer 126 depletes the electrons in the carrier channel 108 under the combined opening.

The semiconductor structure 100 also includes a gate electrode 128 disposed over the combined opening on AlGaN layer 106 between the source and drain features. The gate electrode 128 includes a conductive material layer configured for voltage bias and electrical coupling with the carrier channel 108. In various examples, the conductive material layer may include a refractory metal or its compounds, e.g., titanium (Ti), titanium nitride (TiN), titanium tungsten (TiW) and tungsten (W). In another example, the conductive material layer may include nickel (Ni), gold (Au) or copper (Cu). In one example, the gate electrode 128 is disposed on the portion of the carrier channel depleting layer 126 in the combined opening over the AlGaN layer 106. In another example, the gate electrode 128 is directly disposed on the AlGaN layer 106 in the combined opening. There is no the carrier channel depleting layer 126 in between the gate electrode 128 and the AlGaN layer 106. In the above examples, the gate electrode 128 is at least partially embedded in the protection layer 114 and the dielectric cap layer 110.

The semiconductor structure 100 also includes a depletion region 124 in the carrier channel 108 under the combined opening in the protection layer 114 and the dielectric cap layer 110. The carrier channel 108 becomes normally-off because of the depletion region 124. A positive gate voltage should be applied to turn on the carrier channel 108 of this HEMT. This HEMT is also called an enhanced-mode HEMT that is opposite to a depletion-mode HEMT. The depletion-mode HEMT has a normally-on carrier channel and a negative gate voltage should be applied to turn off the carrier channel.

The semiconductor structure 100 further includes a fluorine-containing region 122 in AlGaN layer 106 under the combined opening (namely under the gate electrode 128). It is believed that fluorine ions in the fluorine-containing region 122 provide strong immobile negative charges and effectively deplete the electrons in depletion region 124.

FIG. 1B is a cross-sectional view of the semiconductor structure 101 having a HEMT according to another embodiment of this disclosure. The layer stacks of the semiconductor structure 101 are similar to the semiconductor structure 100 shown in FIG. 1A. However, the semiconductor structure 101 does not include the fluorine-containing region 122 shown in FIG. 1A. In structure 101, the plasma-deposited carrier channel depleting layer 126 in the combined opening over the AlGaN layer 106 effectively depletes the electrons in depletion region 124.

In the above described embodiments, the gate electrode 128, the source/drain features, and the carrier channel 108 in the GaN layer 104 are configured as a transistor. When a voltage is applied to the gate stack, a device current of the transistor could be modulated.

FIG. 2 is a flowchart of a method 200 of forming a semiconductor structure having a HEMT according to one or more embodiments of this disclosure. Referring now to FIG. 2, the flowchart of the method 200, at operation 201, a first III-V compound layer is provided. The first III-V compound layer is formed on a substrate. Next, the method 200 continues with operation 202 in which a second III-V compound layer epitaxially is grown on the first III-V compound layer. The method 200 continues with operation 203 in which a source feature and a drain feature are formed on the second III-V compound layer. The method 200 continues with operation 204 in which a carrier channel depleting layer is deposited on at least a portion of the second III-V compound layer. The method 200 continues with operation 205 in which a gate electrode is formed over the portion of the second III-V compound layer between the source feature and the drain feature. It should be noted that additional processes may be provided before, during, or after the method 200 of FIG. 2.

FIGS. 3 to 8 are cross-sectional views of the semiconductor structure 100 having a HEMT at various stages of manufacture according to various embodiments of the method 200 of FIG. 2. Various figures have been simplified for a better understanding of the inventive concepts of the present disclosure.

Referring to FIG. 3, which is an enlarged cross-sectional view of a portion of a substrate 102 of a semiconductor structure 100 after performing operations 201, 202 and 203. In some embodiments, the substrate 102 includes a silicon carbide (SiC) substrate, sapphire substrate or a silicon substrate. A first III-V compound layer 104, also refer to as a channel layer, is grown on the substrate 102. In this embodiment, the first III-V compound layer 104 refers to a gallium nitride (GaN) layer (also referred to as the GaN layer 104). The GaN layer 114 can be epitaxially grown by metal organic vapor phase epitaxy (MOVPE) using gallium-containing precursor and nitrogen-containing precursor. The gallium-containing precursor includes trimethylgallium (TMG), triethylgallium (TEG), or other suitable chemical. The nitrogen-containing precursor includes ammonia (NH₃), tertiarybutylamine (TBAm), phenyl hydrazine, or other suitable chemical. The GaN layer 104 has a thickness in a range from about 0.5 micron to about 10 microns. In other embodiments, the first III-V compound layer 104 may include a GaAs layer or InP layer.

A second III-V compound layer 106, also refer to as donor-supply layer, is grown on first III-V compound layer 104. An interface is defined between the first III-V compound layer 104 and the second III-V compound layer 106. A carrier channel 108 of 2-DEG is located at the interface. In at least one embodiment, the second III-V compound layer 106 refers to an aluminum gallium nitride (AlGaN) layer (also referred to as the AlGaN layer 106). The AlGaN layer 106 can be epitaxially grown on the GaN layer 104 by MOVPE using aluminum-containing precursor, gallium-containing precursor, and nitrogen-containing precursor. The aluminum-containing precursor includes trimethylaluminum (TMA), triethylaluminium (TEA), or other suitable chemical. The gallium-containing precursor includes TMG, TEG, or other suitable chemical. The nitrogen-containing precursor includes ammonia, TBAm, phenyl hydrazine, or other suitable chemical. The AlGaN layer 106 has a thickness in a range from about 5 nanometers to about 50 nanometers. In other embodiments, the second III-V compound layer 106 may include an AlGaAs layer, or AlInP layer.

A dielectric cap layer 110 is deposited on a top surface 107 of the second III-V compound layer 106. The dielectric cap layer 110 has a thickness in a range from about 100 Å to about 5000 Å. The dielectric cap layer 110 may include SiO₂ or Si₃N₄. In one example, the dielectric cap layer 110 is Si₃N₄ and formed by performing a low pressure chemical vapor deposition (LPCVD) method without plasma using SiH₄ and NH₃ gases. An operation temperature is in a range of from about 650° C. to about 800° C. An operation pressure is in a range of about 0.1 Torr and about 1 Torr. The dielectric cap layer 110 may protect the underlying second III-V compound layer 106 from damage in the following process having plasma. Two openings in the dielectric cap layer 110 are defined by lithography and etching processes to expose a portion of the second III-V compound layer 106.

A metal layer is deposited over the dielectric cap layer 110, overfills the openings and contacts the second III-V compound layer 106. A photoresist layer (not shown) is formed over the metal layer and developed to form a feature over the openings. The metal layer not covered by the feature of the photoresist layer is removed by a reactive ion etch (RIE) process that etches the exposed portions of the metal layer down to the underlying the dielectric cap layer 110. Metal features 112 are generated after the etching process. The metal features 112 are configured as the source feature or the drain feature for the HEMT. The photoresist layer is removed after the formation of the metal feature. The dielectric cap layer 110 protects the underlying the second III-V compound layer 106 from damage during the etching process to form metal features 112. The carriers in carrier channel 108 of 2-DEG underlying the second III-V compound layer 106 would not be affected. The electrical performances of the semiconductor structure 101 would be positively affected. Therefore, the yield of the overall assembly could increase.

The metal layer of the metal features 112 may include one or more conductive materials. In at least one example, the metal layer is free of gold (Au) and comprises titanium (Ti), titanium nitride (TiN), or aluminum copper (AlCu) alloy. In another example, the metal layer includes a bottom Ti/TiN layer, an AlCu layer overlying the bottom Ti/TiN layer, and a top Ti layer overlying the AlCu layer. The formation methods of the metal layer include atomic layer deposition (ALD) or physical vapor deposition (PVD) processes. Without using Au in the metal features 112, the method 200 could also be implemented in the production line of integrated circuits on silicon substrate. The contamination concern from Au on the silicon fabrication process could be eliminated.

Next, a protection layer 114 is deposited on top surfaces of the metal features 112 and the dielectric cap layer 110. The protection layer 114 may include dielectric materials such as SiO₂ or Si₃N₄. In one example, the protection layer 114 is Si₃N₄ and formed by performing a plasma enhanced chemical vapor deposition (PECVD) method.

FIG. 4 illustrates the structure 100 after forming an isolation region 116 in the first III-V compound layer 104 and the second III-V compound layer 106. The isolation region 116 isolates the HEMT in the structure 100 from other devices in the substrate 102. In one example, the isolation region 116 is formed by implantation process with species of oxygen or nitrogen. The protection layer 114 protects the source feature and the drain feature from being exposed during an anneal process after the implantation process.

FIG. 5 illustrates the structure 100 after forming an opening 118 in the protection layer 114 and the dielectric cap layer 110. A patterned mask layer (not shown) is formed on a top surface of the protection layer 114 and an etching process is performed to remove a portion of the protection layer 114 and the dielectric cap layer 110. The opening 118 exposes a portion of the top surface 107 of the second III-V compound layer 106. The opening 118 is configured as a location for the later gate electrode formation.

FIG. 6 illustrates the structure 100 after treating a portion of the second III-V compound layer 106 with fluorine (F) 120 through the opening 118. The treated portion of the second III-V compound layer 106 forms a fluorine-containing region 122. A concentration of F atoms decay from the top surface 107 of the second III-V compound layer 106 towards the carrier channel 108 by one order magnitude. In one example, the structure 100 is treated in a plasma environment comprising CF₄. An operation power of the plasma environment is about 100 W to about 1000 W. A bias power to guide the plasma radicals toward the structure 100 is less than about 350 W. It is believed that fluorine ions in the fluorine-containing region 122 provide strong immobile negative charges and effectively deplete the electrons in the carrier channel 108. A depletion region 124 in the carrier channel 108 is generated under the fluorine-containing region 122. The HEMT in the structure 100 is converted from a depletion-mode HEMT to an enhanced-mode HEMT. The carrier channel 108 becomes normally-off and a positive gate voltage should be applied to turn on the carrier channel 108 for this enhanced-mode HEMT. In another example, the structure 100 is treated with fluorine by an implantation process. In yet another example, the fluorine-containing region 122 is kept within the second III-V compound layer 106 and does not contact the carrier channel 108 nor further extends into the first III-V compound layer 104. This configuration prevents F atoms from penetrating into the first III-V compound layer 104 and prevents negative effects by the operation of this enhanced-mode HEMT.

FIG. 7 illustrates the structure 100 after depositing a carrier channel depleting layer 126 in operation 204. The carrier channel depleting layer 126 is plasma deposited on the protection layer 114, along an interior surface of the opening 118 and on the exposed portion of the second III-V compound layer 106 (namely on the fluorine-containing region 122). The carrier channel depleting layer 126 is in a thickness range from about 50 nm to about 200 nm. In one example, the carrier channel depleting layer 126 is Si₃N₄ and formed by a plasma enhanced chemical vapor deposition (PECVD) method. An operation temperature is in a range of from about 350° C. to about 500° C. An operation power of the plasma environment is about 100 W to about 600 W. The energetic radicals in the plasma environment contact the exposed portion of the second III-V compound layer 106 and also effectively deplete the electrons in the depletion region 124 of the carrier channel 108. If the operation power is lower than 100 W, the effect to deplete the carrier channel 108 is not obvious. However, if the operation power is higher than 600 W, the whole carrier channel 108 of the HEMT in structure 100 may be depleted. The HMET may not be functional. An acceptable yield of the devices is related to the operation power.

FIG. 8 illustrates the structure 100 after performing operation 205, which forms a gate electrode 128 over the portion of the second III-V compound layer 106 above the depletion region 124. In one example, the gate electrode 128 is formed on a portion of the carrier channel depleting layer 126 above the fluorine-containing region 122. In another example, the carrier channel depleting layer 126 is removed after the deposition process in operation 204. The electrons in the depletion region 124 of the carrier channel 108 are depleted during the deposition process in operation 204 and will not be influenced even if the deposited carrier channel depleting layer 126 is removed. The gate electrode 128 is formed on the portion of the second III-V compound layer 106 without the carrier channel depleting layer 126 in between. In one example, a gate electrode layer is deposited on the carrier channel depleting layer 126 and overfills the opening 118 shown in FIG. 7. Lithography and etching processes are performed on the gate electrode layer to define the gate electrode 128 between the source and drain features. The gate electrode 128 includes a conductive material layer that may include a refractory metal or its compounds, e.g., titanium (Ti), titanium nitride (TiN), titanium tungsten (TiW) and tungsten (W). In another example, the gate electrode 128 may include nickel (Ni), gold (Au) or copper (Cu).

Note that in all of the above embodiments, the structure 100 having a HEMT can be formed through the processes illustrated in FIGS. 3 to 8 and the related text. The processes for forming the structure 101 having a HEMT are similar to the forming of the structure 100 except that the fluorine treatment in FIG. 6 is removed. In FIG. 7, depositing the carrier channel depleting layer 126 using plasma also effectively depletes the electrons in the depletion region 124 in the absence of the fluorine treatment. The structure 101 having an enhanced-mode HEMT can be formed.

Various embodiments of the present invention may be used to improve the performance of a semiconductor structure having a high electron mobility transistor (HEMT). For example, in conventional methods, a portion of the second III-V compound layer 106 is etched to form a recess for an enhanced-mode HEMT. During etching the recess, the etching uniformity among the semiconductor chip regions on the same substrate 102 is hard to control. The electrical performances of each HEMT in the same semiconductor chip region or the same substrate 102 could not be accurately controlled. In this disclosure, the carrier channel depleting layer 126 deposited using plasma depletes the electrons in the carrier channel 108 for an enhanced-mode HEMT. The plasma-deposited carrier channel depleting layer 126 eliminates the drawbacks in conventional methods. The metal feature 112 is free of Au and comprises Al, Ti or Cu. Without using Au in the metal feature 112, the method 200 could also be implemented in the production line of integrated circuits on silicon substrate. The contamination concern from Au on the silicon-Fab process could be eliminated. Compared with the HEMT with Au in source/drain feature, the cost for manufacturing the HEMT according to the present application is reduced. Both the III-V semiconductor compounds process and the silicon-Fab process can be implemented in the same production line. It increases the flexibility to allocate different products for the production line.

An embodiment of the disclosure describes a semiconductor structure. The semiconductor structure includes a first III-V compound layer. A second III-V compound layer is disposed on the first III-V compound layer and different from the first III-V compound layer in composition. A carrier channel is located between the first III-V compound layer and the second III-V compound layer. A source feature and a drain feature are disposed on the second III-V compound layer. A gate electrode is disposed over the second III-V compound layer between the source feature and the drain feature. A carrier channel depleting layer is disposed on the second III-V compound layer. The carrier channel depleting layer is deposited using plasma and a portion of the carrier channel depleting layer is under at least a portion of the gate electrode.

Another embodiment of the disclosure describes a semiconductor structure. The semiconductor structure includes a GaN layer disposed on a substrate. An AlGaN layer is disposed on the GaN layer. A carrier channel is located between the GaN layer and the AlGaN layer. A source feature and a drain feature spaced apart are disposed on the AlGaN layer. A gate electrode disposed over the AlGaN layer between the source feature and the drain feature. A portion of a carrier channel depleting layer disposed between the gate electrode and the AlGaN layer. The portion of the carrier channel depleting layer is overlying a depletion region of the carrier channel.

The present disclosure also describes an embodiment of a method of forming a semiconductor structure. The method includes providing a first III-V compound layer. A second III-V compound layer is epitaxially grown on the first III-V compound layer. A carrier channel is located between the first III-V compound layer and the second III-V compound layer. A source feature and a drain feature are formed on the second III-V compound layer. A carrier channel depleting layer is plasma deposited on a portion of the second III-V compound layer. A gate electrode is formed over the portion of the second III-V compound layer between the source feature and the drain feature.

Although the embodiments and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, and composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps. 

What is claimed:
 1. A semiconductor structure comprising: a first III-V compound layer; a second III-V compound layer disposed on the first III-V compound layer and different from the first III-V compound layer in composition, wherein a carrier channel is located between the first III-V compound layer and the second III-V compound layer; a dielectric cap layer over the second III-V compound layer; a source feature and a drain feature disposed on a top surface of the second III-V compound layer; a gate electrode disposed over the second III-V compound layer between the source feature and the drain feature; a carrier channel depleting layer disposed on the second III-V compound layer, wherein a portion of the carrier channel depleting layer is under at least a portion of the gate electrode; and a protection layer over the dielectric cap layer and over the source feature and contacting a sidewall surface of the source feature farthest from the gate electrode and over the drain feature and contacting a sidewall surface of the drain feature farthest from the gate electrode, wherein the carrier channel depleting layer extends over the protection layer.
 2. The semiconductor structure of claim 1, wherein the carrier channel under the gate electrode comprises a depletion region.
 3. The semiconductor structure of claim 1, wherein the carrier channel depleting layer depletes a portion of the carrier channel.
 4. The semiconductor structure of claim 1 further comprising a fluorine (F) region embedded in the second III-V compound layer under the gate electrode.
 5. The semiconductor structure of claim 1, wherein the carrier channel depleting layer overlies the source feature and the drain feature.
 6. The semiconductor structure of claim 1, wherein the dielectric cap layer underlies the carrier channel depleting layer.
 7. The semiconductor structure of claim 1, wherein each of the source feature and the drain feature is free of Au and comprises Al, Ti or Cu.
 8. The semiconductor structure of claim 1, wherein the second III-V compound layer comprises AlGaN, AlGaAs, or AlInP.
 9. The semiconductor structure of claim 1, wherein the gate electrode comprises titanium (Ti), titanium nitride (TiN), titanium tungsten (TiW), tungsten (W), nickel (Ni), gold (Au) or copper (Cu).
 10. A semiconductor structure comprising: a gallium nitride (GaN) layer disposed on a substrate; an aluminum gallium nitride (AlGaN) layer disposed on the GaN layer, wherein a carrier channel is located between the GaN layer and the AlGaN layer; a dielectric cap layer over the AlGaN layer; a source feature and a drain feature spaced apart and disposed on a top surface of the AlGaN layer; a gate electrode disposed over the AlGaN layer between the source feature and the drain feature; a portion of a carrier channel depleting layer disposed between the gate electrode and the AlGaN layer, wherein the portion of the carrier channel depleting layer is overlying a depletion region of the carrier channel; and a protection layer over the dielectric cap layer and over the source feature and contacting a sidewall of the source feature farthest from the gate electrode and over the drain feature and contacting a sidewall of the drain feature farthest from the gate electrode, wherein the carrier channel depleting layer extends over the protection layer.
 11. The semiconductor structure of claim 10, wherein the carrier channel depleting layer depletes the depletion region of the carrier channel.
 12. The semiconductor structure of claim 10 further comprising a fluorine (F) region embedded in the AlGaN layer under the gate electrode.
 13. The semiconductor structure of claim 10, wherein the carrier channel depleting layer comprises a nitride layer.
 14. The semiconductor structure of claim 10, wherein the dielectric cap layer underlies the carrier channel depleting layer.
 15. The semiconductor structure of claim 10, wherein each of the source feature and the drain feature is free of Au and comprises Al, Ti or Cu.
 16. The semiconductor structure of claim 10, wherein the gate electrode comprises titanium (Ti), titanium nitride (TiN), titanium tungsten (TiW), tungsten (W), nickel (Ni), gold (Au) or copper (Cu).
 17. A method of forming a semiconductor structure, the method comprising: providing a first III-V compound layer; epitaxially growing a second III-V compound layer on the first III-V compound layer, wherein a carrier channel is located between the first III-V compound layer and the second III-V compound layer; forming a source feature and a drain feature on the second III-V compound layer; forming a dielectric layer over the second III-V compound layer; forming a protection layer over the dielectric layer and the source feature and the drain feature, wherein the dielectric layer is between the second III-V compound layer and the protection layer; forming an opening in the protection layer and the dielectric layer between the source feature and the drain feature, wherein a sidewall of the protection layer is substantially aligned with a sidewall of the dielectric layer; plasma depositing a carrier channel depleting layer in the opening in the protection layer and the dielectric layer on a portion of the second III-V compound layer; and forming a gate electrode over the portion of the second III-V compound layer between the source feature and the drain feature, wherein the protection layer is over the source feature and contacting a sidewall of the source feature farthest from the gate electrode, and the protection layer is over the drain feature and contacting a sidewall of the drain feature farthest from the gate electrode.
 18. The method of claim 17 further comprising treating a portion of the second III-V compound layer under the gate electrode with fluorine (F) before depositing the carrier channel depleting layer.
 19. The method of claim 17, wherein forming the gate electrode comprises forming the gate electrode on the carrier channel depleting layer.
 20. The method of claim 17 further comprising removing the carrier channel depleting layer before forming the gate electrode. 